1. Field of the Invention
The present invention relates to a heterogeneous polyvinyl ester dispersion stabilized by protective colloids and to a process for the preparation thereof. The invention also relates to a method of using a heterogeneous polyvinyl ester dispersion stabilized by protective colloids as a solvent- and plasticizer-free adhesive for bonding porous substrates.
2. Description of Related Art
Emulsion adhesives, which are often based on polyvinyl esters, are in practice frequently formulated with low- or high-boiling solvents. In their function as temporary plasticizers, the solvents serve as auxiliaries for film consolidation by lowering the minimum filming temperature or white point of the dispersions. This measure ensures that the adhesives can be processed even at temperatures only just above the freezing point of water. These adhesives have the advantage that they only have a slight adverse effect on the mechanical properties of the adhesive film, but they have a major disadvantage involving the release of solvents into the environment. A list of common solvents suitable for this purpose is provided, for example, in Wood Adhesives, Chemistry and Technology, Volume 1, Marcel Dekker, New York, 1989, pp. 332-333, and in Volume 2, p. 44.
A further function of the aforementioned solvents is to prevent coagulation of the dispersions when the adhesives are stored at low temperatures. The addition of low-boilers, such as methanol, ethanol and acetone, as antifreeze agents to polyvinyl acetate dispersions is described in Chemical Abstracts 86: 56287s.
Another group of relatively high-boiling compounds typically is added to the adhesive dispersions as permanent plasticizers. This group includes, for example, dibutyl phthalate and similar compounds. Although these remain in the film after drying, the heat stability of the adhesive bond and its resistance to cold flow can be impaired as a result of their use. A review of these compounds is likewise provided in Wood Adhesives, Chemistry and Technology, Volume 1, Marcel Dekker, New York, 1989, p. 331 and in Volume 2, p. 43-44.
A solvent- and plasticizer-free adhesive composition is therefore fundamentally of advantage with respect to environmental and user protection, since both toxicologically unacceptable substances and substances which cause an odor nuisance or environmental hazard on use can be omitted completely. It can therefore be used, in particular, in adhesive formulations described as "solvent-free".
Water-based adhesives in which the addition of film-forming auxiliaries or plasticizers is unnecessary due to internal plastification of the polyvinyl acetate by means of suitable comonomers, such as ethylene, are described in the patent literature. For example, DE-C 31 15 601 describes aqueous synthetic resin emulsions for use, for example, in an adhesive, paper-treatment composition or coating material, including polyvinyl acetate emulsion adhesives for wood which have good freezing resistance on storage at low temperatures. An emulsion adhesive for wood based on a copolymer of vinyl acetate and ethylene is also described. Low-temperature stability of the dispersions is principally achieved, without additional solvents, using acetoacetylated polyvinyl alcohols as protective colloids. The disclosure of DE-C 31 15 601 is incorporated by reference herein in its entirety.
DE-A 23 01 099 describes a process for the preparation of adhesive compositions based on terpolymers of vinyl acetate, ethylene and N-methylolacrylamide which are stabilized by partially hydrolyzed polyvinyl alcohol. The disclosure of DE-A 23 01 099 is incorporated by reference herein in its entirety. These polymers have improved resistance to cold flow and increased heat resistance of the adhesive bonds. These properties are achieved by carrying out the polymerization in a specific way, where the vinyl acetate and N-methylolacrylamide monomers are, in a delayed feed process, metered in such a way that the content of unpolymerized vinyl acetate in the batch as a whole does not exceed 1% at conversions of up to 75%.
It also is known to replace, in particular, the solvents used for frost protection by less-volatile compounds. For example, DE-A 38 39 936 describes frost-resistant emulsion adhesives based on polyvinyl esters which contain amides of aliphatic carboxylic acids and/or of carbamic acid, in particular acetamide. However, these formulations do not additionally contain plasticizers such as dibutyl phthalate or butyl diglycol acetate.
JP-A 78 541/93 describes polyvinyl acetate dispersions as wood and paper adhesives having a low filming temperature without addition of plasticizers. However, these compositions contain substantial amounts, for example 16% by weight, based on vinyl acetate, of a copolymer of isobutene and maleic anhydride as protective colloid in addition to polyvinyl alcohol.
DE-A 27 18 716 describes contact emulsion adhesives based on an ethylene-vinyl acetate copolymer that can be modified by the addition of an emulsion of an acrylate copolymer. However, the known high price of acrylates means that this modification considerably increases the cost of this emulsion adhesive.
JP-A 65550/94 describes an adhesive composition comprising a mixture of two vinyl acetate-ethylene copolymer dispersions with ethylene contents in the copolymer of, in each case from 5 to 40% by weight, of which one component is stabilized by means of polyvinyl alcohol and the other by means of nonionic emulsifiers. The presence of ethylene in both components and the presence of emulsifiers in at least one component results in adhesive bonds whose heat resistance is unsatisfactory.
A particular problem when using copolymers of vinyl acetate and ethylene which are stabilized using polyvinyl alcohol and which are prepared by conventional pressure emulsion polymerization processes with copolymerization with adequate amounts of ethylene to obtain the desired MFT values of below 7.degree. C. is that the heat resistance of conventional polyvinyl ester homopolymer dispersion films containing film-forming auxiliaries is not achieved (see Comparative Examples V2 and V3 compared with comparative example V1 described herein). The documents described herein are incorporated by reference in their entirety.